1. Field
Embodiments of the present invention relate in general to disc shaped recording media, and more particularly to alignment a disc storage media within a disc drive unit.
2. Description of the Related Art
Hard disc drives provide prerecorded tracking servo information on data recording surfaces of their magnetic hard discs. This servo information is typically spaced evenly along tracks. Data is recorded between the servo information. In some cases, servo information is radially aligned, and looks like spokes of a wheel.
During operation, the disc drive magnetic read/write head flies over the spinning discs and reads information from the tracks as the information passes beneath the head. This information includes both data and servo information. The servo information tells the disc drive servo system where the head is in relation to the track in order that the disc drive servo system may adjust the head's radial position to keep the head on track center.
The servo information may be written onto a disc's surface using a variety of techniques. A newer approach mounts a large number of discs onto a spinstand and writes the servo information onto all of them at the same time. This approach, also known as “multiple disc write” (“MDW”), is attractive because a number of discs maybe written at the same time. However, this approach also introduces a number of problems because the discs themselves are not assembled onto the disc drive spindle at the time servo tracks are written.
One of the problems is that the disc must be mounted onto the drive spindle in the same “orientation” it was written. For example, if the servo tracks were written by a rotary actuator, the servo burst will be arrayed in an arc that follows the arc of the servo-track writer. To read such servo information properly, the disc drive's actuator should be aligned to traverse the same arc as the servo-writer. For this to occur, the discs are mounted in the same orientation for the disc drive's actuator as they were for the servo-writer's actuator. In most cases, this is accomplished by assuring that a disc's “top” surface when it is mounted in the servo-writer also be the “top” surface when that same disc is mounted in the disc drive.
A second problem occurs when two (or more) such prerecorded MDW discs are mounted in a single disc drive. Not only should the arcs of both discs be aligned, but also the tracks themselves should be substantially aligned vertically, that is, the tracks of one disc should be substantially congruent with tracks of the other discs such that they form “aligned” cylinders. In this manner, the disc drive can switch between a head reading a specific track number on the top surface of the topmost disc, to a head reading the same track number on of the top surface of the next disc in the stack without needing to perform a seek. If the respective tracks were significantly misaligned in some fashion, switching between them without performing a seek or some other alignment technique would not be possible. After a head switch, the drive may need to find out its location. This may even require it to seek a track “0” in order to recover a proper track number.
A third problem caused by writing servo data while the disc is mounted on a different spindle is track eccentricity. A hard disc's inner diameter has a tolerance specification much looser than the track eccentricity specification of most disc drive servo systems. If the disc is mounted on the disc drive's spindle in a manner that is significantly off center from the way it was written on the servo writer, it could exceed the drive's servo system eccentricity tolerance. While most disc drive servo systems have eccentricity feed forward mechanisms to help increase their eccentricity tolerances, these feed forward systems typically have stroke limits far below the loosest tolerances for hard disc inner diameters.
Another problem is the angular alignment of the servo spokes from one disc to another. If the angular misalignment is too large, the drive cannot reliably switch heads from one disc surface to another located on a second disc.
A current practice for achieving such alignment is to bias all the discs in the MDW servo-writer against the servo-writer's spindle so that each of the discs' inner diameters is vertically aligned at a point of contact with the servo-writer's spindle. Thereafter, when these discs are assembled onto a disc drive's spindle, alignment of their inner surface contact points that abut against the disc drive spindle causes them to have the same vertical alignment they had in the MDW servo-writer. Depending on the precision of alignment of the discs in both the MDW servo-writer and in the disc drive, and upon the precision that the respective servo information is written by the MDW servo-writer, the alignment of the inner surface contact points automatically aligns the tracks of respective discs congruently into the same cylinders they had on the servo-track writer.
A conventional way to align the discs onto both the servo-track writer and disc drive spindles so that the same portion of the disc inner surface contacts each is to mark the discs with a laser prior to their being loaded onto the MDW servo-writer. However, the laser marking tools are expensive and bulky, they require substantial clean room space and different tools are required to mark glass and nickel phosphorus/aluminum substrates. Finally, the use of a laser marking tool can adversely impact drive reliability in a number of ways.
One approach to solving these problems with using laser markings was to record the alignment marks magnetically when the servo track data is written. Commonly assigned U.S. Pat. Nos. 7,221,528 and 6,940,678 describe the use of magnetic alignment marks, also referred to as magnetic index marks (MIMs), to indicate the location on a magnetic recording surface of a magnetic hard disc where the disc abuts against the spindle of a multiple-disc servo writer. MIMs are especially well suited for use with magnetic media in which the disc head can be used to write the alignment marks.
Patterned magnetic media refers to hard disc recording media produced using techniques to achieve higher recording density by printing individual grains onto the magnetic media. Using these techniques, a data bit can be stored on a single grain, resulting in greater disc storage density. More specifically, Discrete Track Recording (DTR) or Bit Patterned Media (BPM) processes can be used to imprint islands (tracks for DTR) onto a substrate of a data storage disc. To produce the islands, a template is formed. Once the template is created, it can be used in a process to produce patterned media.
It is appreciated that analogous alignment problems persist for patterned magnetic media. Producing alignment marks with lasers is undesirable for the reasons set forth above. Moreover, producing MIMs is impractical for patterned magnetic media in which data is encoded magnetically using nanolithography techniques. Thus, there has been a need to produce alignment marks for printed magnetic media.